The major object of this project is to uncover enzymatic steps involved in various genetic rearrangment reactions and to study the mechanism of action of the enzymes involved. The reactions studied include: the site specific recombination of bacteriophage lambda, resolution of the Holliday intermediate of general genetic recombination transposition-replication reaction of bacteriophage Mu, and rearrangement reaction of immunoglobulin genes. The most recent developments include the establishment of an in vitro reaction system for the study of replicative transposition of bacteriophage Mu. This in vitro reaction yields both cointegrate products as well as simple insertion products and requires both A and B gene products of phage Mu along with other bacterial proteins. The reaction requires a substrate DNA molecule that carries two ends of Mu in their proper relative orientation. These conditions are also the requirements for the Mu transposition-replication reaction in vivo. Thus, the in vitro reaction observed is the authentic Mu tarnsposition. This is the first time a transposition reaction has been demonstrated in a cell-free system. By making use of this reaction system, we have obtained evidence that support the mechanism of transposition involving a pair of strand transfer reactions as the initiation step. The polarity of this strand transfer reaction has been determined. MuA and B gene products as well as the Mu repressor that is involved in the regulation of the process have been purified to near homogeneity. MuA protein was shown to bind specifically to the two end sequences of Mu DNA. Mu repressor was shown to bind specifically to the operator region of Mu DNA. Functional interaction between the two proteins were investigated.